Circuit breakers are well known in the field of high and medium voltage switching devices. They comprise nominal contacts and arcing contacts used to open and close the electrical connection. During the opening and closing operations of the circuit breaker the current commutates between the nominal and the arcing contacts and vice-versa, respectively. For these operations at least one nominal contact and one arcing contact move with respect to their respective mating contacts. It is also possible that both nominal and arcing contacts move towards one another.
During the closing procedure first the arcing contacts are connected and after that the nominal contacts. Typical configurations of nominal contacts involve a tube or a rod as a first nominal contact and contact fingers arranged in a so-called finger cage (typically rotational symmetric to a longitudinal axis of the circuit breaker) as the second nominal contact. Consequently, the tube or rod is driven into the finger cage and its outer surface contacts the contact fingers.
During the contacting process the contact fingers have an impact point, in which the first mechanical contact to the tube or rod occurs, whereafter the contact fingers slide on the surface of the tube or rod up to a contact point on their surface, which represents the electrical connection when the circuit breaker is in a contact-closed (i.e. conducting) state. The impact point and the contact point are different from one another.
When the contact fingers are very close to the tube or rod, current starts to flow through the two nominal contacts and small arcs are formed at the impact point of the contacts. These arcs create commutation marks in the vicinity of the impact point, which can extend up to the contact point, thus damaging the contacts. Even though the contact fingers can still be used for subsequent operations, they increasingly wear out in the area of the contact point with every new commutation operation.